Light conversion efficiency enhanced solar cell fabricated with downshifting nanomaterial

ABSTRACT

The light conversion efficiency of a solar cell ( 10 ) is enhanced by using an optical downshifting layer ( 30 ) in cooperation with a photovoltaic material ( 22 ). The optical downshifting layer converts photons ( 50 ) having wavelengths in a supplemental light absorption spectrum into photons ( 52 ) having a wavelength in the primary light absorption spectrum of the photovoltaic material. The cost effectiveness and efficiency of solar cells platforms ( 20 ) can be increased by relaxing the range of the primary light absorption spectrum of the photovoltaic material. The optical downshifting layer can be applied as a low cost solution processed film composed of highly absorbing and emissive quantum dot heterostructure nanomaterial embedded in an inert matrix to improve the short wavelength response of the photovoltaic material. The enhanced efficiency provided by the optical downshifting layer permits advantageous modifications to the solar cell platform that enhances its efficiency as well.

RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/225,472, filed Jul. 14, 2009.

COPYRIGHT NOTICE

© 2010 Spectrawatt, Inc. A portion of the disclosure of this patentdocument contains material that is subject to copyright protection. Thecopyright owner has no objection to the facsimile reproduction by anyoneof the patent document or the patent disclosure, as it appears in thePatent and Trademark Office patent file or records, but otherwisereserves all copyright rights whatsoever. 37 CFR §1.71(d).

TECHNICAL FIELD

This disclosure relates to solar cell devices and, in particular, to asolar cell having a photoabsorptive nanomaterial layer that downshiftscertain wavelengths of incident light into a wavelength region that isbetter absorbed by the photovoltaic material of the solar cell.

BACKGROUND INFORMATION

“First generation” solar cells are based on the use of crystallinesilicon as the photovoltaic material. These silicon solar cells, whichhave light conversion efficiency of about 16-21%, currently have thehighest light conversion efficiency of solar cells in high-volumeproduction (excluding high-cost III-V inorganic cells intended for usein outer space). However, the cost of electricity from these siliconsolar cells is still higher in dollar per watt than most currentlyavailable retail peak electricity rates. The cost of electricity perwatt generated by a solar cell can generally be changed in either of twoways: the light conversion efficiency of the solar cell can beincreased, or the cost of producing the solar cell can be decreased.

FIG. 1A is a graph showing the spectral irradiance versus wavelength,and FIG. 1B shows the maximum fraction of general spectral irradianceutilized by silicon. Due to the band gap of silicon, the maximumfraction of light that can be used is about 44%. Due to thermodynamicsand recombination inside the silicon material, the maximum practicalattainable light conversion efficiency is about 26%.

The concept of enhancing solar cell performance by employing spectralconverters has been proposed. Downconversion techniques are disclosed byTrupke, T. and Green, M. A., “Improving solar cell efficiencies bydown-conversion of high energy photons,” Journal of Applied Physics, v.92, no. 3, Pgs. 1668-1674 (2002) and by Richards, B. S., “Enhancing theperformance of silicon solar cells via the application of passiveluminescence conversion layers,” Solar Energy Materials & Solar Cells90, Pgs. 2329-2337 (2006). Downconversion generally refers to theabsorption of one high energy photon and the subsequent emission ofmultiple low energy photons, such that overall photon energy isconserved.

Also, the concept of absorption and re-emission of one photon, with theloss to heat of the difference in energy between photons (hereafterreferred to as “downshifting,” not to be confused with downconversion)has been proposed. Batchelder, J. S. et al., “Luminescent SolarConcentrators: 1) Theory of Operation and Techniques for PerformanceEvaluation,” Applied Optics 18, Pgs. 3090-3110 (1979) and Currie, M. J.,et al., “High Efficiency Organic Solar Concentrators for Photovoltaics,”Science 321, Pgs. 226-228 (2008) describe past wavelength conversionapproaches, which have been based largely on dyes that have very highluminescence quantum efficiencies but have limited spectral tunabilityand degrade rapidly. Such limited spectral tunability and rapiddegradation are discussed by Kinderman, R., et al., “I-V Performance andStability of Dyes for Luminescent Plate Concentrators,” Journal of SolarEnergy Engineering 129, Pgs. 277-282 (2007). Dyes also typically have avery large overlap between their absorption and emission spectrum, suchthat self-absorption losses are significant.

Use of a CdSe quantum dots (QD) as a converting material has beenproposed by Van Sark, W. G. J. H. M., “Enhancement of Solar CellPerformance by Employing Planar Spectral Converters,” Applied PhysicsLetters 87, 151117 (2005).

Other materials that have been tested as wavelength converters includesilicon nanocrystals, such as those described by Svrcek, V., et al.,“Silicon Nanocrystals as Light Converter for Solar Cells,” Thin SolidFilms 451-452, Pgs. 384-388 (2004). These materials also havedisadvantages.

More cost-effective solar cell technology is, therefore, stilldesirable.

SUMMARY OF THE DISCLOSURE

The light conversion efficiency of a solar cell can be increased byemploying a downshifting nanomaterial to supplement the activity of aphotovoltaic material.

In some of embodiments, a photovoltaic material is wafer-based.

In some of embodiments, a photovoltaic material is tailored to emphasizeabsorption in a wavelength region emitted by the downshifting material.

In some of embodiments, the wavelength absorption range of aphotovoltaic material is relaxed in wavelength regions that are notemitted by the downshifting material or in wavelength regions of poorabsorbance of the photovoltaic material.

In some embodiments, the solar cell platform is modified to enhancecollection or absorption of wavelength-specific photons within thedownshifting material at the expense of absorption of the samewavelength-specific photons within the solar cell platform.

In some of embodiments, the production cost of the solar cell platformis favored over its light conversion efficiency.

In some embodiments, the downshifting material includes a tunablenanomaterial.

In some embodiments, the nanomaterial is a quantum dot heterostructure(QDH).

In some embodiments, the quantum dot heterostructure is amulti-component nanocrystal which has been specifically tailored insize, chemical composition, and shape to be a downshifting material.

In some embodiments, the nanomaterial or quantum dot heterostructure hasa non-spherical shell.

In some embodiments, the nanomaterial or quantum dot heterostructure isindividually encapsulated.

In some embodiments, the downshifting material is solution-processible,i.e., the material is created in solution and can be applied as aliquid.

In some embodiments, the solution processed material is stabilized in amatrix and the matrix is deposited as a film.

In some embodiments, a solar cell employing a downshifting material isused as a low cost rooftop device.

In some embodiments, enhanced efficiency provided by the opticaldownshifting layer permits advantageous modifications to the solar cellplatform that enhances its efficiency as well.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph showing the spectral irradiance versus wavelength.

FIG. 1B is a graph showing the spectral irradiance generally availablefor collection and the fraction of the spectral irradiance effectivelyabsorbed by silicon.

FIG. 1C is a graph showing the spectral irradiance generally availablefor collection, the fraction of the spectral irradiance effectivelyabsorbed by silicon, and an exemplary fraction of the spectralirradiance that could be used for downshifting.

FIG. 2A is an enlarged simplified cross-sectional view of an exemplarysolar cell including a photovoltaic material and an optical downshiftinglayer.

FIG. 2B is an enlarged portion of the cross-sectional view of FIG. 2Ashowing absorption and emission occurring in the optical downshiftinglayer.

FIG. 3 is simplified enlarged drawing of an encapsulated nanomaterial,such as a quantum dot heterostructure.

FIG. 3A is a TEM image of an exemplary CdSe/CdS quantum dotheterostructure.

FIG. 3B is a TEM image of an exemplary CdSe/CdS quantum dotheterostructure grown to a different aspect ratio than that of FIG. 3A.

FIG. 3C is a TEM image of exemplary encapsulated quantum dotheterostructures: CdSe dot cores having rod-shaped CdS shellsindividually encapsulated in a silica encapsulating material.

FIG. 3D is a TEM image of an exemplary manganese-doped ZnSe quantumdots.

FIG. 4 is a graph of absorption and emission versus wavelength, showingabsorption and emission by exemplary quantum dot heterostructures.

FIG. 5 is a graph of exemplary external quantum efficiency (EQE) of asilicon solar cell versus wavelength, showing the EQE differencesbetween solar cells with and without an optical downshifting layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Within the fixed portion spectral irradiance from the Sun, silicon has aregion of light conversion activity from about 0.3 micron to about 1.2microns due to the 1.1 eV band gap of silicon for absorbing incidentphotons and generating electron and hole charge carriers, as shown inFIG. 1B. For silicon, this region is characterized at least in part by aprimary light absorption spectrum from about 0.6 micron to about 1micron, in which silicon effectively absorbs photons and generateselectron and hole charge carriers. FIG. 1B shows, however, that siliconis far less effective at absorbing sunlight in about the 0.2 micron toalmost 0.6 micron spectral region. Other photovoltaic materials exhibitareas of the spectral region that are less effective at absorbingsunlight than in their respective primary light absorption spectra.

For a typical solar cell platform using conventional photovoltaicmaterial and having a limited primary light absorption spectrum, theefficiency of the solar cell platform can be increased by adding aspectral shifting layer. The spectral shifting layer shifts incidentspectral radiation having wavelengths in a supplemental light absorptionspectrum, which is substantially outside of the primary light absorptionspectrum, to photons having wavelengths in the primary light absorptionspectrum. The spectral shifting layer is also transmissive to incidentphotons having wavelengths in the primary light absorption spectrum andallows them to pass into the photovoltaic material so that thephotovoltaic material can directly absorb these incident photons.

For example, in some embodiments, the spectral shifting layer is anoptical downshifting layer that can absorb photons having higher bandgap energies (shorter wavelengths) that are not readily absorbed by thephotovoltaic material and emit photons having lower band gap energies(longer wavelengths) within primary light absorption spectrum that arereadily absorbed by the photovoltaic material. Thus, a solar cell can bemade to absorb the spectral irradiance more efficiently by employingsuch a spectral shifting layer in conjunction with the photovoltaicmaterial. FIG. 1C is a graph showing the spectral irradiance generallyavailable for collection, the fraction of the spectral irradianceeffectively absorbed by silicon, and an exemplary fraction of thespectral irradiance that could be used by an optical downshifting layer.

FIG. 2A is an enlarged simplified cross-sectional view of an exemplarysolar cell 10 including a solar cell platform 20 having a photovoltaicmaterial 22 positioned between an emitter electrode 24 and a rearcontact electrode 26, which electrodes are spaced-apart from each other.(Emitter electrode 24 may also be referred to as top, upper, front, orsun-facing electrode 24, and rear contact electrode 26 may also bereferred to as bottom, lower, back, or earth-facing electrode 26.)Photovoltaic material 22 includes at least one charge-separatingjunction 27 where differently doped regions of bulk photovoltaicmaterial 22 meet. Although p-n type charge-separating junctions 27 aremost common, n-p type, p-i-n type, and other type junctions may beemployed. Skilled persons will also appreciate that even thoughcharge-separating junction 27 is shown as a line (with a planarinterface texture and no volume), charge-separating junction 27 may havea nonplanar interface texture and may include a volume (including aheight) of bulk material due to doping gradients. In some embodiments,photovoltaic material 22 and emitter and rear contact electrodes 24 and26 form respective charge-separating junction interfaces 28 and 29 forseparating the electron and hole charge carriers for collection byemitter electrode 24 and rear contact electrode 26.

With reference to FIG. 2A, emitter electrode 24 directly or indirectlysupports an optical downshifting layer 30. In some embodiments, opticaldownshifting layer 30 is deposited directly on a textured externalsurface 34 of emitter electrode 24. In some embodiments, one or moresupplemental layers 32, such as an anti-reflective coating (ARC), may bepositioned on or applied to external surface 34 of emitter electrode 24between emitter electrode 24 and optical downshifting layer 30. Frontgrid contacts 38 may extend from emitter electrode 24 throughsupplemental layers 32 and optical downshifting layer 30 to conveyelectrical current from emitter electrode 24 to the surface of solarcell 10.

FIG. 2B is an enlarged portion of the cross-sectional view of FIG. 2A,showing absorption and emission occurring in optical downshifting layer30. The components of optical downshifting layer 30 are not drawn toscale. In particular, optical downshifting layer 30 absorbs a photon 50having a wavelength in the supplemental light absorption spectrum ofphotovoltaic material 22 and emits a photon 52 having a wavelength inthe primary light absorption spectrum of photovoltaic material 22.

Photovoltaic material 22 can be any material that actively absorbs lightand converts it to electricity. For example, photovoltaic material 22can be any conventional photovoltaic material such as a first, second,or third generation photovoltaic material. Exemplary photovoltaicmaterials 22 include, but are not limited to, crystalline silicon (c-Si)or multicrystalline silicon (mc-Si); nanocrystalline silicon; amorphoussilicon (a-Si); micromorphous silicon; gallium arsenide (GaAs); InP;InAs; combinations of GaAs, InP, or InAs; other III-V-based photovoltaicmaterials; cadmium telluride (CdTe), copper indium selenide (CIS),copper indium gallium di-selenide (CIGS), active organic photovoltaicmaterials, dye-sensitized solar cell (DSC) materials, and active quantumdot (QD) ensembles. Quantum dot ensembles are disclosed in detail inU.S. patent application Ser. No. 12/606,908, entitled Solar CellConstructed with Inorganic Quantum Dots and Structured MolecularContacts, which is herein incorporated by reference. In someembodiments, photovoltaic material 22 forms a wafer-based solar cellplatform 20.

In some embodiments, any photovoltaic material 22 having limitedspectral activity in the blue wavelength region could be employed. Insome embodiments, silicon-based photovoltaic materials 22 have a primarylight absorption spectrum from about 0.6 micron to about 1 micron. Insome embodiments, CdTe-based photovoltaic materials 22 have a primarylight absorption spectrum from about 500 nm to about 900 nm.

In terms of efficiency, a photovoltaic material 22 preferably has alight conversion efficient of at least 10%. In some embodiments, a lowerlight conversion efficiency may be adequate if the production costs arelow. In some embodiments, a moderately or highly efficient photovoltaicmaterial 22 is preferred, especially in area-constrained applications.In particular, some preferred photovoltaic materials 22 have lightconversion efficiency of greater than 15%.

In some embodiments, photovoltaic material 22 is selected based on lowproduction costs. In many embodiments, photovoltaic material 22 isselected based on the lowest production cost for the highest efficiency.In many embodiments, other selection factors, such as longevity andreliability, as well as primary absorption spectrum, are also oralternatively considered for choosing a suitable photovoltaic material22. For example, a typical solar cell module based on silicon-containingsolar cell platforms has a 25-year warranty. Thus, desirable longevityfor photovoltaic material 22 may be even longer.

Because the effect of downshifting layer 30 is additive, the lightconversion efficiency of photovoltaic material 22 is not critical.Although practical optimized light-conversion efficiency is preferred,even a photovoltaic material 22 with non-optimal light conversionefficiency can be used as a platform. Such platforms with suboptimalefficiency may be intentionally selected by design or not. For example,a solar cell platform 20 having a photovoltaic material 22 generated ina screen-printed process can be used because the blue part of thespectrum, where a screen-printed solar cell platform 20 is mostinefficient, will be downshifted by downshifting layer 30.

In some embodiments, optical downshifting layer 30 has a sharp, tunableabsorption onset at a wavelength where the external quantum efficiency(EQE) of photovoltaic material 22 begins to drop. In some embodiments,the supplemental light absorption spectrum is substantially outside ofthe primary light absorption spectrum of the active solar device so asto avoid reduction of light conversion activity of the photovoltaicmaterial in the primary light absorption spectrum of the active solardevice. In some embodiments, the majority of the supplemental lightabsorption spectrum is outside of the primary light absorption spectrum.In some embodiments, the supplemental light absorption spectrum isentirely outside of the primary light absorption spectrum.

In some embodiments, the onset absorption wavelength falls between 500nm and 600 nm. Thus, in some examples, the supplemental light absorptionspectrum of optical downshifting layer 30 may include wavelengthsshorter than or equal to 600 nm. In some embodiments, the tunableabsorption spectrum ranges downward to include the lower range ofsignificant spectral irradiance. In some embodiments, the tunableabsorption spectra ranges downward to include wavelengths of 300 nm orshorter. In some examples, the supplemental light absorption spectrum ofoptical downshifting layer 30 may include wavelengths longer than orequal to 200 nm.

In some embodiments, optical downshifting layer 30 has an emissionspectrum that is separated from the supplemental light absorptionspectrum such that the emitted light is not reabsorbed in opticaldownshifting layer 30. In some embodiments, the emission spectrum isseparated from the supplemental light absorption spectrum by greaterthan 50 nm or by an energy gap of greater than 0.5 eV. In someembodiments, optical downshifting layer 30 has an independently tunableemission spectrum. In some embodiments, optical downshifting layer 30has an emission spectrum that emits photons having a wavelength greaterthan 600 nm. In some embodiments, optical downshifting layer 30 has anemission efficiency of greater than 90%. In some embodiments, opticaldownshifting layer 30 is simple and inexpensive to lay down on top ofthe solar cell platform 20, using a quick, solution-based process atambient pressure.

In some embodiments, optical downshifting layer 30 includes one or morenanomaterials to efficiently absorb in the supplemental light absorptionspectrum and emit photons in the primary light absorption spectrum of agiven photovoltaic material 22, i.e., the nanomaterial forming opticaldownshifting layer 30 absorbs photons where the spectral absorbance ofphotovoltaic material 22 is low and reemits photons at a wavelengthwhere the spectral absorbance of photovoltaic material 22 is high.

Nanomaterials are materials with at least one nano-scale dimension, areoften grown colloidally, and have been made in the form of dots, rods,tetrapods, and even more exotic structures. (See Scher, E. C.; Manna,L.; Alivisatos, A. P. Philosophical Transactions of the Royal Society ofLondon. Series A: Mathematical, Physical and Engineering Sciences 2003,361, 241 and Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.;Alivisatos, A. P. Nat Mater 2003, 2, 382-385.) Their sizes generallyrange from 3 nm to 500 nm. Due to the quantum size effects which arisefrom a material having dimensions on the order their electron's bohrradius, the band gap of the material can also be tuned (See Alivisatos,A. P. J. Phys. Chem. 1996, 100, 13226-13239 and Bawendi, M. G.;Steigerwald, M. L.; Brus, L. E. Annual Review of Physical Chemistry1990, 41, 477-496.) In addition to facilitating tunability of the bandgap for absorption and emission, the nanomaterials often have nearperfect crystallinity, allowing them to attain extremely highphotoluminescence (See Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.;Aloni, S.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2007, 7, 2951-2959and Xie, R.; Battaglia, D.; Peng, X. J. Am. Chem. Soc. 2007, 129,15432-15433.)

The nanomaterials can be suspended in an inert polymer matrix materialthat could also be used as the encapsulating material for solar cellplatform 20. Nanomaterials are highly suitable for use as opticaldownshifting layer 30 and offer serious advantages over dyes. Thenanomaterials are solution processible, highly controllablesemiconductor nanostructures synthesized by low-cost solution-basedmethods and can be made to have the exact optical properties desired foroptical downshifting layer 30. Because of their unique structure andcomposition, nanomaterials can be more stable than dyes.

Semiconductor nanocrystals have unique physical and electronicproperties due to their quantum confinement. Because of their highsurface area to volume ratios, the optical and electrical properties ofsemiconductor nanocrystals are highly governed by their surface defects.In a homogeneous dispersion of nanocrystals in a matrix, there is astrong nanocrystal-nanocrystal interaction induced by the hydrophobicityof the organic ligand, possibly leading to aggregation.

In some embodiments, optical downshifting layer 30 includesnanomaterials, particularly quantum dot heterostructures (QDHs),encapsulated discretely by secondary materials through a micelleapproach. FIG. 3 is simplified enlarged drawing of an encapsulatedquantum dot heterostructure 60. With reference to FIG. 3, encapsulatedquantum dot heterostructure 60 includes a quantum dot heterostructurehaving a core 62 surrounded by one or more shells 64. Shell 64 isfurther encapsulated by an encapsulating material 66, such as a silicasphere.

By discretely encapsulating each semiconductor nanocrystal, it ispossible to homogeneously disperse the nanocrystals in a matrix media,as well as protect the nanocrystal surface from the externalenvironment. Therefore, the introduction of the encapsulating materials66 greatly helps to both passivate nanocrystal surface defects andisolate the individual nanocrystals for better dispersion. Thus, theencapsulating materials 66 minimize the interaction among thenanocrystals, improving the stability as well as the homogeneity in amatrix media.

For such optical downshifting materials 30, outer encapsulatingmaterials 66 can be grown on individual nanocrystals non-epitaxially.Micelles are formed using a pair of polar and non-polar solvents in thepresence of a compatible surfactant. The surface polarity of ananocrystal can be modified so that only a single nanocrystal willreside in an individual micelle. Subsequently, an inorganic or organicencapsulating material 66 may be selectively grown inside of themicelle, which acts as a spherical template. By adding additionalprecursors, an inorganic or polymeric encapsulating material 66 can befurther grown on the nanocrystal. (See Selvan, S. T.; Tan, T. T.; Ying,J. Y. Adv. Mater., 2005, 17, 1620-1625; Zhelev, Z.; Ohba, H.; Bakalova,R. J. Am. Chem. Soc., 2006, 128, 6324-6325; and Qian, L.; Bera, D.;Tseng, T.-K.; Holloway, P. H. Appl. Phys. Lett., 2009, 94, 073112.)

Thus, by tuning the synthetic conditions, a single nanocrystal can beincorporated in a silica sphere as shown in FIG. 3 (and FIG. 3C).Throughout the encapsulation process, the nanocrystal surfaces are wellpassivated to avoid aggregation. Additionally, this passivation endowsthe nanocrystals with photoluminescence quantum yields of and nearunity. For encapsulated nanocrystals, the matrix compatibility can bedependent on the surface of the encapsulating sphere, not thenanocrystal. Since the surface of the encapsulating material 66 isspatially removed from the nanocrystal surface, alterations to theexterior of the encapsulating material 66 do not adversely affect theelectronic or optical properties of the nanocrystal.

Semiconductor nanocrystals, such as cadmium selenide or indiumphosphide, have widely been studied for control over both theircomposition and shape. (See Scher, E. C.; Manna, L.; Alivisatos, A. P.Philosophical Transactions of the Royal Society of London. Series A:Mathematical, Physical and Engineering Sciences 2003, 361, 241 andTalapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase,M.; Weller, H. J. Am. Chem. Soc 2002, 124, 5782-5790.)

Thus, in addition to spherically-shaped nanostructures, variousnon-spherical nanostructures have been demonstrated including, but notlimited to, nanorods, nanotetrapods, and nanosheets. Non-sphericalsemiconductor nanocrystals have different physical and electronicproperties from those of spherical semiconductor nanocrystals. Theseproperties can be employed advantageously in downshifting optical layer30.

In some embodiments, the nanomaterials are quantum dot heterostructures.A quantum dot heterostructure is a nanomaterial which has beenspecifically engineered, including but not limited to, tailoring insize, chemical composition, shape, optical, and/or electricalproperties, to perform a particular function. In particular, the quantumdot heterostructures can be multi-component nanocrystals tailored fordownshifting applications.

In some embodiments, optical downshifting material 30 may includeindividually encapsulated nanomaterials, particularly quantum dotheterostructures, employing one type of core material, one type(composition) of shell material, and one shape of shell material. Insome embodiments, optical downshifting material 30 may includeindividually encapsulated quantum dot heterostructures, particularlyquantum dot heterostructures, employing two or more varieties ofindividually encapsulated quantum dot heterostructures, such as a firsttype of individually encapsulated quantum dot heterostructure, employinga first type of core material, a first type of shell material, and afirst shape of shell material and a second type of individuallyencapsulated quantum dot heterostructure, employing the first type ofcore material, the first type of shell material, and at least one ormore different shapes of shell material, such as rods and tetrapods.

In some embodiments, the second type of individually encapsulatedquantum dot heterostructure employs a first type of core material, atleast one or more different types of shell material, such as ZnS or CdS,and the first or at least one or more different shapes of shellmaterials. In such embodiments, each shell material may be associatedwith a specific shape, or each shell material may be formed with aplurality of shapes. In some embodiments, the second type individuallyencapsulated quantum dot heterostructures employs at least one or moredifferent types of core materials, the first or one or more differenttypes of shell materials, and the first or one or more different typesof shell shapes. In such embodiments, each core material may beassociated with specific shell materials and/or shapes, or each corematerial may be associated with one or more shell materials and/orshapes.

As described above, some advantages of employing a quantum dotheterostructure as an optical downshifting layer 30 include an increasedsolar cell efficiency in the blue part of the solar spectrum, wheretypical photovoltaic materials 22 have a low quantum efficiency. The QDHoptical downshifting layer 30 also provides a decreased thermal load inthe solar cell platform 20 because the blue photons are absorbed aboveand never reach photovoltaic material 22, where most of them would beconverted to heat inside solar cell platform 20.

In one example, the onset of absorption and separation betweenabsorption and emission peaks can be tuned by 0.5 eV or more, enablinglong wavelength light to pass through unhindered to solar cell platform20 beneath and minimizing re-absorption within optical downshiftinglayer 30. Pradhan, N., Peng, X., “Efficient and Color-Tunable Mn-DopedZnSe Nanocrystal emitters: Control of Optical Performance via GreenerSynthetic Chemistry,” J. Am. Chem. Soc., 129 (11), Pgs. 3339-3347(2007); Talapin, D. V., Nelson, J. H., Shevchenko, E. V., Aloni, S.,Sadtler, B., Alivisatos, A. P., “Seeded Growth of Highly LuminescentCdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies,”Nanoletters v. 7, no. 10 Pgs. 2951-2959 (2007); and Sholin, V., Olson,J. D., Carter, S. A. “Semiconducting polymers and quantum dots inluminescent solar concentrators for solar energy harvesting,” Journal ofApplied Physics, v. 101, no. 12, Pg. 123114 (2007) describe methods fortuning nanomaterials.

In some embodiments, the quantum dot heterostructures can includefollowing inorganic compounds and/or any combination of alloys betweenthem: CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, CuS₂, CuSe₂, In₂S₃, In₂Se₃,CuGaSe₂, CuGaS₂, CuInS₂, CuInSe₂, PbSe, PbS, SiO₂, TiO₂, ZnO, ZrO. Thesematerials can be arranged in cores 62, core-shells, andcore-shell-shells, with or without organic ligands, such as phosphonicacids, carboxylic acids, amines.

In some examples, quantum dot heterostructures including CdSe, CdSe/ZnS,CdSe/CdS, or CdTe have provided very high luminescence. Quantum dotheterostructures based on the II-VI chalcogenides are very wellunderstood as high efficiency emitters. In solution, the quantum dotheterostructures particles have quantum efficiencies as high as 95%. Thequantum dot heterostructure materials may be distributed in matrices ofpolydimethylsiloxane, polyvinylbutyral, or ethylvinylacetate, forexample, and may be incorporated into the encapsulating material andcover both emitter electrode 24 and rear contact electrode 26.

FIG. 3A is a transmission electron microscopy (TEM) image of anexemplary CdSe/CdS quantum dot heterostructure. FIG. 3B is a TEM imageof an exemplary CdSe/CdS quantum dot heterostructure grown at atemperature lower than that of FIG. 3A. In some embodiments, the core62, such as a CdSe core 62 may have no physical dimension greater than 6nm, while the extended shell 64, such as a CdS extended shell 64, has atleast one dimension greater than 15 nm and another dimension that isabout 1-2 nm thicker than the CdSe core 62. Thus, the thickness of theshell 64 may be as thin as 1 or 2 nm along some axes and may be greaterthan 9 nm along other axes. The casing of the oxide encapsulatingmaterial 66 can range between 1 and 50 nm in radius.

In some embodiments, quantum dot heterostructure materials of opticaldownshifting layer 30 exhibit a tunable onset of absorption at awavelength of around 600 nm, with a range of approximately 50 nm. In allcases, the absorption rapidly increases moving to the ultraviolet. Thegap between absorption onset and peak emission in the quantum dotheterostructure materials of optical downshifting layer 30 may also betuned between 50 nm to 200 nm in separation.

In some embodiments, quantum dot heterostructure materials of opticaldownshifting layer 30 exhibit supplemental light absorption rangeshaving supplemental light absorption spectrum with a maximum absorptionwavelength at about 600 nm and a minimum absorption wavelength in theultraviolet. In some embodiments, the supplemental light absorptionspectrum of the QDH materials covers a range of at least 250 nm. In someembodiments, the range of the supplemental light absorption spectrum maybe shorter, and the maximum absorption wavelength may also be shorter toprovide for a gap between absorption and emission in the QDH materialsof optical downshifting layer 30. In some embodiments, the minimumemission wavelength is about 50 to 200 nm longer than the maximumabsorption wavelength.

In one example, optical downshifting layer 30 having quantum dotheterostructure material absorbs photons at a maximum absorptionwavelength of about 480 nm and emits photons at a minimum emissionwavelength of about a 620 nm wavelength. In some embodiments, theemission will be slightly shifted more into to the red spectrum. Forexample, using a CdTe core material instead of CdSe provides red shiftedemission.

In some examples, optical downshifting layers 30, including a quantumdot heterostructure of CdSe quantum dots with a graded shell of CdS thattransitions into ZnS, have maximum absorption wavelengths between about500 and 700 nm and have supplemental light absorption spectrumsextending to shorter wavelengths. In some embodiments, the minimumemission wavelength is about 50 to 100 nm longer than the maximumabsorption wavelength. In one example, optical downshifting layer 30,having a CdSe-based quantum dot heterostructure material, has a maximumabsorption wavelength of about 450 nm and a minimum emission wavelengthof about 550 nm, with a re-emission efficiency of greater than 80%.

In some examples, optical downshifting layer 30 includes quantum dotheterostructures of CdSe quantum dots with a rod-shaped CdS shell 64,encapsulated in a silica encapsulating material 66. This quantum dotheterostructure material exhibits maximum absorption at wavelengthsshorter than 500 nm and maximum emission at wavelengths between 550-700nm. The silica shell serves the dual purpose of enhancing thepassivation of the nanocrystals for increased photoluminescencere-emission efficiency, while also preventing nanocrystal aggregationand increasing the dispersability of the nanocrystal in the downshiftinglayer matrix. FIG. 3C is a TEM image of exemplary quantum dotheterostructures of CdSe quantum dot cores 62 having rod-shaped CdSshells 64 individually encapsulated in a silica encapsulating material66.

In some embodiments, optical downshifting layer 30 includes CdSe quantumdots with a rod-shaped CdS shell 64, which is further covered by asecond shell 64 of ZnSe, which is encapsulated in a silica encapsulatingmaterial 66.

Non-Cd based quantum dots can also be employed. In one example, dopedZnSe has been used in matrices of polydimethylsiloxane,polyvinylbutyral, or ethylvinylacetate. In some examples, the ZnSe isdoped with manganese. FIG. 3D is a TEM image of an exemplary ismanganese-doped ZnSe quantum dot heterostructure.

In some examples, ZnSe quantum dots have a maximum absorption wavelengthbetween 350 and 450 nm. The minimum emission wavelengths largely dependon the nature of the particular structures but range from a shift ofabout 100 to 200 nm longer than the maximum absorption wavelength. Insome examples, InP quantum dot heterostructure materials have a maximumabsorption wavelength ranging from 500 to 750 nm, with minimum emissionwavelengths that are longer.

FIG. 4 is a graph of absorption and emission versus wavelength, showingexemplary quantum dot heterostructure activity with absorption in asolid line and emission in a dashed line. Skilled persons willappreciate that the concept of employing quantum dot heterostructuresfor use as downshifting layer 30 is not dependent on the specificmorphology of the solar cells platforms 20 or its junctions. Thelabeling on the left of the graph shows relative absorbance, and thelabeling on the right of the graph shows relative photoluminescence.

To illustrate the expected efficiency increase due to downshifting, thelosses associated with downshifting have been calculated to determinethat a maximum EQE of about 80% is achievable in the absorbing window ofoptical downshifting layer 30. Light emitted within the escape cone ofoptical downshifting layer 30 is primarily responsible for the 80%limit.

FIG. 5 is a graph of exemplary EQE versus wavelength, showing the EQEdifferences between a solar cell with and without optical downshiftinglayer 30. The thick line shows the typical EQE response of amulticrystalline solar cell platform 20 (made at ECN in the Netherlands)in the absence of optical downshifting layer 30, and the thin line showsthe EQE in the presence of optical downshifting layer 30.

In one embodiment, an optimal design will choose an optical downshiftinglayer 30 with an absorption onset (maximum absorption wavelength) at thewavelength the EQE of photovoltaic material 22 drops below 80%. In oneexample, the photovoltaic material 22 is multicrystalline silicon.Consequently, the EQE would be 80% or higher at the low end of theprimary light absorption spectrum of multicrystalline silicon. Theexample shown in FIG. 5 presents an increased conversion efficiency ofabout 0.5% absolute, due solely to the downshifting effect.

While a 0.5% increase in conversion efficiency may appear to be aminimal improvement, incorporation of optical downshifting layer 30allows the design rules for solar cell platform 20 and its photovoltaicmaterial 22 to be relaxed, such as by decoupling the short wavelengthresponse from the long wavelength response.

Skilled persons will appreciate that the use of a blue-absorbing opticaldownshifting layer 30 will work advantageously with a cell technologywhose external quantum efficiency drops sharply in the blue.Traditionally, solar cell platforms 20 based on silicon photovoltaicmaterials 22 have been optimized to have a spectral response over thewidest possible wavelength range at the expense of wavelength specificresponse. So, by attempting to capture some blue light with theseconventional photovoltaic materials 22 or conventional solar cellplatforms 20, the resulting conventional solar cell platforms 20 havediminished capacity in red regions of their spectral response.

By employing an optical downshifting layer 30 that performs best atshort wavelengths, a solar cell designer can tailor the solar cellplatform 20 or its photovoltaic material 22 to increase the response incertain visible and near infrared (NIR) wavelengths at the expense ofits performance in the blue.

In some embodiments, the structure of multicrystalline silicon in solarcell platform 20 can be modified in order to optimally pair it with ablue-absorbing optical downshifting layer 30.

In some embodiments, at least a 1-2% absolute gain in light conversionefficiency is possible with small modifications, such as lowering theresistance of the emitter electrode 24 to the front grid contact 38, andsuch as tuning the optics of solar cell platform 20 to respond better tolonger wavelength light. Therefore, an optimally integrated solar cell10 having a blue-absorbing optical downshifting layer 30 and a modifiedphotovoltaic material 22 (or its environment) can have a lightconversion efficiency increase of greater than 1% absolute. Thisincrease can be accomplished at an additive cost of 8-10 cents for eachwafer, for example. The concurrent increase in efficiency and poweroutput would, however, lower the overall cost per watt of a standardmulticrystalline silicon solar cell 20, for example, by roughly 40 centsper watt.

In some embodiments, external surface 34 of emitter electrode 24 ofsolar cell platform 20 is modified to increase the light conversionefficiency in the primary absorbance spectrum. In some examples, thedoping level (such as total amount of dopant) of emitter electrode 24 ismodified (typically increased) to reduce series resistance. In someexamples, the emitter electrode 24 can possess a sheet resistance oforder 5-30 Ohms/square (such as a surface doping concentration ofgreater than 1e²⁰/cm³ and an emitter thickness ranging from 1.0-0.5 μm),representing a drastic reduction from the level of 65-110 Ohms/square(such as a surface doping concentration of 1-2e²⁰/cm³ to about 5e¹⁹/cm³and an emitter thickness ranging from 0.4-0.2 μm) for conventional solarcells that do not have an optical downshifting layer 30. For example,higher phosphorous surface concentration results in a lower specificcontact resistance for a tunneling based carrier transport mechanism. Athicker 5-30 ohms/sq emitter would not only lead to reduced solar cellshunting issues and hence a higher yield but also provide a wider solarcell process window. It is also noted that dopant density at the emittersurface decreases toward the p-n junction. For example, a phosphorusdensity of about 2e²⁰/cm³ at the emitter surface decreases to about1e¹⁷/cm³ at the p-n junction.

In some examples, the increased doping level emitter electrode 24 mayreduce the blue response of solar cell platform 20 because a more highlydoped emitter electrode 24 may become less transmissive to blue photons.However, the blue photons, which are absorbed by optical downshiftinglayer 30 at a much greater efficiency than they can be absorbed byphotovoltaic material 22, are downshifted into the primary absorbancespectrum. The downshifted photons are not significantly adverselyaffected by the increased doping emitter electrode 24 and make their wayinto photovoltaic material 22, where they are readily absorbed andconverted in holes and carriers that are captured by emitter electrode24 (and/or rear contact electrode 26). In some examples, a gas or liquidmixture containing phosphorous can be used to dope p-type Si and convertit to n-type emitter electrode 24. Conventional thicknesses of theemitter electrode are 200-400 nm, but in embodiments employing opticaldownshifting layer 30, the emitter electrode 24 can be as little as200-400 nm, but also thicker than 400 nm and as thick as 1 micron. Inparticular, higher doping can easily yield specific contact resistancevalues that are less than 3 mohm-cm2 to achieve good fill factors.

Skilled persons will appreciate that doping and other parameters of rearcontact electrode 26 may also be modified.

In some embodiments, the doping level of emitter electrode 24 ismodified (typically increased) and the gap between front grid contacts38 is increased, thereby decreasing the fractional shading of theunderlying layers. The increased doping level of emitter electrode 24compensates for increased resistivity incurred by the increased gapbetween front grid contacts 38, so the overall resistance of the solarcell platform 20 is about the same (or not significantly adverselyaffected). However, the increased gap between front grid contacts 38reduces shading of photovoltaic material 22 so that more of it isexposed to ambient light, thereby increasing the capture efficiency ofphotons incident on solar cell 10. For example, the spacing between gridcontacts 38 can be increased from roughly 2 mm to greater than 3 mm.This increased gap has the effect of decreasing the number of gridcontacts 38 needed and thereby decreasing the grid electrode shadingpercentage from greater than 6% to less than 5%. Although the increaseddoping of emitter electrode 24 decreases the response of the solar cellplatform 20 to blue wavelengths, for example, optical downshifting layer30 more than adequately compensates for the loss, as previouslyexplained.

The width 80 of front grid contacts 38 can additionally or alternativelybe reduced to reduce the shading of photovoltaic material 22 so thatmore of it is exposed to ambient light, thereby increasing the captureefficiency of photons incident on solar cell 10. The increased dopinglevel of emitter electrode 24 can compensate for increased resistivityincurred by the decreased cross-sectional area of front grid contacts38, so the overall resistance of the solar cell platform 20 is about thesame (or not significantly adversely affected).

In some embodiments, other aspects of front grid contacts 38 arechanged, such as configuration, spacing, height, or other features, tooptimize contact resistance. For example, the height of front gridcontacts 38 can be reduced to decrease shading of photovoltaic material22 by offsetting the decreased resistivity with increased doping ofemitter electrode 24, such as previously described.

With reference again to FIGS. 2A and 2B, a supplemental layer (notshown) or coating material that is reflective to some or all of thewavelengths falling outside the primary light absorption spectrum may beapplied to external surface 34 of emitter 24. Thus, photons 52 withwavelengths in the supplemental light absorption spectrum that reachexternal surface 34 or the supplemental layer may be reflected backinto, and thereby have a second opportunity to be absorbed by, opticaldownshifting layer 30.

In some embodiments, rear contact electrode 26 has an external surface56 that may be coated with a material that is reflective to one or bothof the primary light absorption and the supplemental absorptionspectrum. Thus, in these spectra the photons that reach rear contactelectrode 26 can be reflected back into photovoltaic material 22 andoptical downshifting layer 30 with an opportunity to be absorbed.

In some embodiments, the supplemental layer, such as a passivationlayer, can be added above optical downshifting layer 30 to act in placeof or in addition to, supplemental layer 32, allowing opticaldownshifting layer 30 to act as an anti-reflective coating (ARC) insteadof nitride, for example. The anti-reflective coating tool is currentlythe most expensive piece of equipment in the production line of standardsolar cell platforms 20 and requires the most maintenance and associatedinfrastructure. Replacing the traditional anti-reflective coating with alow cost layer would measurably impact the cost of the producing solarcell platform 20 and the overall cost of solar cell 10. The passivationlayer associated with optical downshifting layer 30 can replicate theeffect of the anti-reflective coating to avoid the loss of efficiencyotherwise afforded by the anti-reflective coating.

In some embodiments, a quantum dot heterostructure optical downshiftinglayer 30 can, itself, offer a lower reflectance and enhancedanti-reflective properties to solar cell platform 20 due to employmentof an intermediate index of refraction in the matrix material containingthe quantum dot heterostructures.

Additional processing and material costs for thin film quantum dotheterostructures as described above are low due to the low-temperature,solution-based deposition processes utilized. Therefore, the price ofelectricity per watt from solar cells 10 having a QDH opticaldownshifting layer 30 is substantially lower than the price of thosewithout one.

Finally, lifetime concerns associated with the quantum dotheterostructure materials for optical downshifting layer 30 aresubstantially mitigated, since along with photovoltaic material 22,optical downshifting layer 30 undergoes the standard and well-understoodencapsulation process into modules. As long as the QDH opticaldownshifting materials themselves are fundamentally stable, the solarcell 10 will have an operating lifetime that is comparable to theoperating lifetimes (30+ years) of conventional silicon solar cellplatforms 20.

A typical solar cell module based on silicon-containing solar cellplatforms 20 has a 25-year warranty, so any additive or replacementmaterial preferably has a similar longevity or at least a comparableone. Alternatively, the additive material can be shorter lived if itprovides adequate benefit during its lifespan and is not detrimentalthereafter. For example, if optical downshifting layer 30 lasts only 10years, but contributes meaningfully to the power generation for those 10years, such lifetime would be sufficient as long as optical downshiftinglayer did not block (such as become nontransmissive to wavelengths inthe primary absorption spectrum) the solar cell platform 20 fromperforming its function after the first ten years. However, in thecontext of 25 years, a 1-2 year lifetime would not long be enough formost embodiments.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. For example,skilled persons will appreciate that subject matter revealed in anysentence, paragraph, or embodiment can be combined with subject matterfrom some or all of the other sentences, paragraphs, or embodimentsexcept where such combinations are mutually exclusive or inoperable. Thescope of the present invention should, therefore, be determined only bythe following claims.

1.-27. (canceled)
 28. An optical downshifting material, comprising: aliquid or matrix medium; and a plurality of encapsulated quantum dotheterostructures (QDHs) disposed in the liquid or matrix medium, eachQDH comprising: a nanocrystalline core comprising a first semiconductormaterial; a nanocrystalline shell comprising a second, different,semiconductor material at least partially surrounding thenanocrystalline core; and an encapsulating coating surrounding thenanocrystalline core and nanocrystalline shell pairing.
 29. The opticaldownshifting material of claim 28, wherein the material has an onsetabsorption wavelength approximately in the range of 500-600 nm.
 30. Theoptical downshifting material of claim 29, wherein the material has alight emission wavelength greater than approximately 600 nm.
 31. Theoptical downshifting material of claim 30, wherein the material has amaximum absorption wavelength of approximately 480 nm, and has a maximumemission wavelength of approximately 620 nm.
 32. The opticaldownshifting material of claim 28, wherein the medium is a matrix mediumconfigured for application to a receiving surface in the form of a film.33. The optical downshifting material of claim 32, wherein the matrix isan inert polymer material matrix.
 34. The optical downshifting materialof claim 28, wherein the medium is a liquid medium configured forapplication to a receiving surface in the form of a solution.
 35. Theoptical downshifting material of claim 28, wherein the encapsulatingcoating passivates the nanocrystalline core and nanocrystalline shellpairing of each QDH.
 36. The optical downshifting material of claim 28,wherein the encapsulating coating protects the nanocrystalline core andnanocrystalline shell pairing of each QDH from the liquid or matrixmedium.
 37. The optical downshifting material of claim 28, wherein theencapsulating coating of each QDH is a silica coating.
 38. The opticaldownshifting material of claim 28, wherein the encapsulating coating ofeach QDH has a radius approximately in the range of 1-50 nm.
 39. Theoptical downshifting material of claim 28, wherein the firstsemiconductor material of each QDH is selected from the group consistingof zinc sulfide (ZnS) and cadmium sulfide (CdS).
 40. The opticaldownshifting material of claim 28, wherein the first semiconductormaterial of each QDH is cadmium selenide (CdSe) and the secondsemiconductor material is cadmium sulfide (CdS).
 41. The opticaldownshifting material of claim 40, wherein the CdSe nanocrystalline coreof each QDH has no dimension greater than approximately 6 nm, and theCdS nanocrystalline shell has at least one dimension greater thanapproximately 15 nm and a second dimension approximately 1-2 nm thickerthan a dimension of the CdSe nanocrystalline core.
 42. A method offabricating an optical downshifting material, the method comprising:forming a plurality of encapsulated quantum dot heterostructures (QDHs),each QDH comprising: a nanocrystalline core comprising a firstsemiconductor material; a nanocrystalline shell comprising a second,different, semiconductor material at least partially surrounding thenanocrystalline core; and an encapsulating coating surrounding thenanocrystalline core and nanocrystalline shell pairing; and suspendingthe plurality of QDHs in a liquid or matrix medium.
 43. The method ofclaim 42, wherein each QDH is formed by first forming a micelle aroundeach nanocrystalline core and shell pairing and then growing theencapsulating coating inside of the micelle.
 44. The method of claim 42,wherein the medium is a matrix medium, the method further comprising:applying, in the form of a film, the matrix medium having the pluralityof QDHs suspended therein to a receiving surface of a device.
 45. Themethod of claim 42, wherein the medium is a liquid medium, the methodfurther comprising: applying, in the form of a solution, the liquidmedium having the plurality of QDHs suspended therein to a receivingsurface of a device.
 46. The method of claim 42, wherein suspending theplurality of QDHs in the liquid or matrix medium comprises protecting,by the encapsulating coating, the nanocrystalline core andnanocrystalline shell pairing of each QDH from the liquid or matrixmedium.
 47. The method of claim 42, wherein forming each of theplurality of encapsulated QDHs comprises forming a cadmium selenide(CdSe) nanocrystalline core, forming a cadmium sulfide (CdS)nanocrystalline shell at least partially surrounding the CdSenanocrystalline core, and forming a silica coating surrounding the CdSenanocrystalline core and CdS nanocrystalline shell pairing.